The present invention relates to a pretensioner that increases a force for restraining an occupant with a seat belt by rotating a spool in a belt retracting direction at an early stage of an emergency when a vehicle undergoes a rapid deceleration of a predetermined level or more due to a collision or the like, a seat belt retractor that has such a pretensioner so as to prevent belt withdrawal in the event of an emergency, and a seat belt device that has such a seat belt retractor so as to restrain an occupant with the seat belt in the event of an emergency.
Conventional seat belt devices installed on vehicles such as automobiles, block the inertial movement of an occupant from a seat by restraining the occupant with a seat belt in the event of an emergency as described above. The seat belt device has a seat belt retractor that allows a seat belt to retract and withdraw using a spool and also locks the seat belt by preventing spool rotation in the belt withdrawal direction with a locking mechanism during the emergency.
As such a seat belt retractor, many seat belt retractors having pretensioner have been proposed, wherein the pretensioner increases a force for restraining an occupant with a seat belt by rotating a spool in a belt retracting direction at an early stage of an emergency (for instance, refer to Publications 1 and 2). Each pretensioner described in the Publications 1 and 2 has a gas generator that is activated to generate gas in the event of an emergency; a piston that is activated with the gas so as to generate a driving force; a plurality of balls for transmitting the driving force generated at the piston to a spool of the seat belt retractor for retracting the seat belt; a pipe for guiding the piston and the plurality of balls; and a spring for urging the piston and the plurality of balls in the same direction as the piston's operating direction. In this case, the piston is formed in a ball shape that is the same as that of the plurality of the balls used to transmit the driving force.
Conventionally, pretensioners are generally assembled after mounting a gas generator on one end of a pipe, by sequentially inserting a spring, a piston, and balls into the pipe from the other end of the pipe. In this case, it is proposed that the gas generator is mounted in the pipe by housing the gas generator in a housing portion that has an expanded diameter on one end where the gas generator is mounted and also by press-fitting the one end (for example, see Publication 3). In the mounting method of the gas generator described in the Publication 3, after press-fitting one end, a circular rib is formed at the press-fitted part; the press-fitted portion is reinforced with the circular rib and the gas generator is prevented from moving out of the pipe while gas is being generated.